Ligand effect on switching the rate-determining step of water oxidation in atomically precise metal nanoclusters

The ligand effects of atomically precise metal nanoclusters on electrocatalysis kinetics have been rarely revealed. Herein, we employ atomically precise Au25 nanoclusters with different ligands (i.e., para-mercaptobenzoic acid, 6-mercaptohexanoic acid, and homocysteine) as paradigm electrocatalysts to demonstrate oxygen evolution reaction rate-determining step switching through ligand engineering. Au25 nanoclusters capped by para-mercaptobenzoic acid exhibit a better performance with nearly 4 times higher than that of Au25 NCs capped by other two ligands. We deduce that para-mercaptobenzoic acid with a stronger electron-withdrawing ability establishes more partial positive charges on Au(I) (i.e., active sites) for facilitating feasible adsorption of OH– in alkaline media. X-ray photo-electron spectroscopy and theoretical study indicate a profound electron transfer from Au(I) to para-mercaptobenzoic acid. The Tafel slope and in situ Raman spectroscopy suggest different ligands trigger different rate-determining step for these Au25 nanoclusters. The mechanistic insights reported here can add to the acceptance of atomically precise metal nanoclusters as effective electrocatalysts.

boosting the electrocatalytic activity of metal NCs 14,15 . The change in superatomic electronic configuration and the follow-up positivelyshifted reduction potentials account for the activity enhancement. Besides metal core, protecting ligands can also significantly influence the electrocatalytic performance of metal NCs 16,17 . In particular, ligands work as outmost layer of metal NCs, directly interacting with the reaction environment (e.g., electrolyte, solvent, and reactant ions/ molecules). Moreover, they determine the electronic structures of metal NCs through the atomic orbitals coupling between the anchoring atoms (e.g., sulfur) and metal atoms [17][18][19] . Considerable research efforts have been devoted to revealing the ligand effects on the electronic structures in terms of the change of HOMO-LUMO gap, which affects the adsorption on reactants or reaction intermediates 17,20 . Nevertheless, an understanding toward the ligand effect on the electrocatalysis rate-determining step (RDS) is currently lacking at the molecular and atomic levels. This should be largely attributed to the intrinsic difficulty in capturing the key reaction intermediates in the electrocatalytic reactions in the presence of [Au 25 (SR) 18 ] -NCs capped by varied SR ligands, although the identification of the key intermediates is crucial to understand the electrocatalytic pathways with different one-step reaction kinetics.
Herein, we implement [Au 25 (SR) 18 ] -NCs separately capped by three different thiolate ligands with similar size (approximated by their similar molecule weights) but different structures (i.e., paramercaptobenzoic acid (pMBA), 6-mercaptohexanoic acid (MHA), and homocysteine (HCys) as model NCs (Fig. 1a), and study the ligand effect on electrocatalytic OER kinetics (e.g., rate-determining step) of [Au 25 (SR) 18 ] -NCs. It has been increasingly known that the Au(I) in the protecting shell of [Au n (SR) m ] q NCs is the active sites for feasible adsorption of reactants/intermediates in diverse catalytic reactions 21 . We therefore hypothesize that SR ligands with similar size, but varied electron-withdrawing capability should be able to induce varied electronegativity of Au(I) in [Au 25 (SR) 18 ] -NCs. The diversity in electronegativity of Au(I) can thus lead to varied impacts on adsorption of the involved reactants with negative charges. Our systemic study reveals that the varied electron-withdrawing capability of pMBA, MHA and HCys can induce distinct changes in the RDS of OER. Specifically, Au NCs capped by ligands with stronger electron-withdrawing ability could develop more local partial positive charge of Au(I) in the Au(I)-SR motif of NCs, facilitating feasible adsorption of OHon the active Au(I) sites in the alkaline media 22 . Such favorable adsorption of OHcan lower the activation energy barrier for intermediates, and thereby altering the RDS of OER 23 .

Au 25 NCs synthesis and characterization
[Au 25 (SR) 18 ]was chosen as model cluster in this study. This is because of its good stability, synthesis feasibility, and known crystal structure [24][25][26] . As shown in Fig. 1a, [Au 25 (SR) 18 ] -NCs possess a Au(0) @Au(I)-SR core-shell structure with quasi-D 2h symmetry, where a 13atom Au(0) core is capped by six staple-like SR-[Au(I)-SR] 2 motifs [27][28][29] . The development of the formal positive charge on the Au atoms in the protecting shell should be largely attributed to the electronwithdrawing capability of protecting SR ligands. Therefore, deliberate control over electron-withdrawing capability of R moiety in SR ligands can offer a good means for fine tuning the electronegativity of Au(I) in the protecting shell. Three representative water-soluble ligands with similar molecular weight (M w = 154 for pMBA, M w = 135 for HCys, and M w = 148 for MHA) but different electron withdrawing ability (pMBA ˃ HCys ˃ MHA) are selected, to exemplify the ligand effects on the OER performance of [Au 25 (SR) 18 ] -NCs. The p-π electron delocalization between benzene ring and sulfur atom renders pMBA with stronger electron-withdrawing ability than the other two ligands 30 . HCys has stronger electron-withdrawing ability than that of MHA, which is interpreted by the XPS spectra of S 2p in the Supplementary Fig. 1. The positive shift of S 2p from MHA to HCys, to pMBA manifests the electron-withdrawing capability of sulfur in ligands 31 .
The Au 25 NCs were synthesized based on a mild-reduction strategy reported by our group 32,33 . The detailed synthetic procedures are provided in the Supplementary Information. These as-synthesized Au   18 ] -NCs as an example (Fig. 1c), there are three sets of peaks observed in a broad range of m/ z = 500-4000, which account for Au 25 NCs capped by MHA carrying 6, 5, and 4 negative charges, respectively. The experimental (black) and simulated (red) isotope patterns of [Au 25 (MHA) 18 −5H] 5show excellent match (inset of Fig. 1c [3][4][5], which are in good agreement with those measured by X-ray crystallography 34 . In order to shed light on the ligand effects on the electronic structures of Au(I) in Au NCs, we carried out X-ray photo-electron spectroscopy (XPS) measurement. The high-resolution XPS spectra of Au 4f of Au NCs were depicted in Fig. 1f, which manifests a dramatically positive shift of Au 4f binding energy for [Au 25 (pMBA) 18

OER performance in alkaline media
The electrocatalytic OER activities of the synthesized Au 25 NCs were evaluated using a standard three-electrode system in a H-type cell (Supplementary Fig. 6) containing O 2 -saturated 1.0 M KOH electrolyte. All potentials are reported versus reversible hydrogen electrode (RHE). Representative linear sweep voltammetry (LSV) curves for OER were recorded with a scan rate of 5 mV s -1 (Fig. 3a). A typical current density of 10 mA/cm 2 was recorded at an overpotential of 360 mV for [Au 25 (pMBA) 18 ] -NCs, which is lower than that of [Au 25 (HCys) 18 ] -NCs (470 mV) and [Au 25 (HCys) 18 NCs] -(540 mV). The pristine carbon paper shows minimal anodic current within the same potential range, eliminating the impact of substrate on the improved electrocatalytic performance. Moreover, the LSV curve comparison for [Au 25 (pMBA) 18 ] -NC and Au-pMBA complex suggests the dominant role of hierarchical cluster structure for effective electrocatalysis ( Supplementary Fig. 7). To address the merits of discrete electronic structures for Au NCs, we also compared their electrocatalytic performance with that of larger Au nanoparticles capped by pMBA (NPs,~5 nm in diameter, prepared by NaBH 4 reduction ( Supplementary Fig. 8). The LSV curves in Supplementary Fig. 9 indicated that Au NPs deliver the highest current density of 2.5 mA/cm 2 within the same potential window from 1.1 to 1.8 V. The weak bonding of OHon gold surface may be responsible for the unsatisfactory activity for the Au NPs 35 . Moreover, we also compared our Au 25 NCs with other thiolate-protected transition metal electrocatalysts in terms of the overpotential required for reaching a current density of 10 mA/cm 2 ( Supplementary Fig. 10   To evaluate the OER kinetics, we further investigated the Tafel plot derived from LSV curves. The Tafel equation formulates a mathematical relationship between the current density and the applied potential, which is vital to understand electrochemical kinetics 38 18 ] -NCs deliver a higher Tafel slope of 304 and 196 mV/dec, respectively. These values imply that the RDS is dominated by deprotonation on Au-OH 39 . The above-mentioned different Tafel slope values suggest the RDS is changed by varying the protecting ligands from MHA or HCys to pMBA. The larger Tafel slope reflects stronger polarization as the current density rises. Such higher Tafel slope value presumably has a variety of causes. On the one hand, Tafel slope varies depending on different RDS. On the other hand, the higher activation energy on intermediates possibly makes Tafel slope value higher 40 18 ] -(20.20 mF/cm 2 ), indicating their superior kinetics ( Supplementary Fig. 17). Moreover, electrochemical impedance spectroscopy (EIS) on [Au 25 (SR) 18 ] -NCs was conducted to infer the charge-transfer resistance. [Au 25 (pMBA) 18 ] -NCs feature the smallest semi-circular arc, indicating their faster electrode kinetics (Fig. 3c). In addition, compared with traditional turnover frequency (TOF) calculation involved electrocatalytic surface area and active site numbers in cell unit, the well-defined molecular structure enables Au 25 NCs to provide a more reasonable understanding towards the intrinsic electrocatalytic performance due to their precise active site numbers. The TOF value of [Au 25 (pMBA) 18 ] -NCs is nearly 4 times higher than that of [Au 25 (MHA) 18 ]and [Au 25 (HCys) 18 ] -NCs, further confirming their better intrinsic activity (Fig. 3d). In order to correlate the Au charge deficiency of Au 25 NCs to their TOF in OER, we determined the Au(I)/Au(0) ratio in individual Au 25 NCs by deconvoluting Au 4f peak in their XPS spectra. Then we plotted TOF vs. Au(I)/ Au(0) ratio in Supplementary Fig. 18. As can be seen in Supplementary  Fig. 18, an ascending trend is observed for TOF with increasing Au(I)/ Au(0) ratio, in good consistency with electron-withdrawing capability of examined thiolate ligands. The volume of generated oxygen in the OER process for [Au 25 (pMBA) 18 ] -NCs was recorded by online trace gas analysis system-gas chromatography (Fig. 3e). A linear relationship matches well with the computed cumulative charge-volume ratio, demonstrating the cathodic FE of [Au 25 (pMBA) 18 ] -NCs is as high as 99.5%. The long-term stability of electrocatalysts is also crucial for their practical implementations 42,43 . As shown in Fig. 3f, [Au 25 (pMBA) 18 ] -NCs exhibit a practically invariant current density within the applied potential window from 1.57 to 1.66 V, suggesting their good electrochemical stability. The strong π-π stacking interactions of pMBA should account for the good structural and thus electrocatalytic stability of corresponding Au 25 NCs. By contrast, [Au 25 (HCys) 18 ] -NCs maintain its current density of 96% in 10 h (Supplementary Fig. 19). The current density of [Au 25 (MHA) 18 ] -NCs degrade to the 60% of the initial value, which may result from the oxidative degradation of [Au 25 (MHA) 18 ] -NCs ( Supplementary Fig. 20) 44 . In addition, we conducted additional experiment to confirm the structural stability of   18 ] -NCs for OER using in situ Raman spectroscopy. A confocal Raman microscope with 532 nm excitation was used to record these spectra ( Supplementary  Fig. 27). The in situ Raman analysis was carried out at various applied potentials around the onset potential. The Raman signals were recorded in a spectral window from 550 to 850 cm -1 , which covers almost all reported oxygen species bonded to Au atom. As shown in Fig. 4a 18 ] -NCs shows an emerging peak at 800 cm -1 with increasing applied potentials (Fig. 4c).The peak at 800 cm -1 is assignable to Au-O-OH species, since similar peak at 815-830 cm -1 was documented as O-O vibration of Au-O-OH in Au electrode system 35,45 . It has been widely accepted that OER current depends on electrocatalytic reaction rate. The reaction rate is closely associated with the coverage of intermediate species. Therefore, the coverage for the intermediates will be affected by the applied potential, and a threshold value is required for a specific intermediate coverage to trigger its extensive conversion 46 . Specifically, if an intermediate specie participates in the RDS as reactant, it will accumulate on the surface of metal NCs, boosting its surface coverage. This also provide an opportunity for us to capture the intermediates using in situ Raman spectroscopy. Although the peak positions vary with the protecting ligand changed from aromatic (i.e., pMBA) to aliphatic (i.e., MHA and HCys), the peak intensity shows a ubiquitous ascending trend with the increase of the applied potentials regardless of the capping agent used (Fig. 4d), suggesting that the coverage of intermediate species, either Au-OH or Au-O-OH, increases with the elevated overpotential. Combined with Tafel slope analysis on different RDS for Taking the varied key intermediates into consideration, it can be proposed that the RDS of OER on Au NCs is highly related to their capping ligands (Fig. 5). To better understand the ligand effects, we adapt a typical OER pathway in the alkaline medium for [Au 25 (SR) 18 18 ] -. Such correlations of adsorption energy with OER activities have been observed by others as well 48,49 . It should also be noted that the OER catalyzed by Au NCs possess very different pathways from those of bulk Au. On the bulk Au surface, it is widely accepted that the adsorbed OH -(Au-OH ads ) will be converted into Au 2 O 3 via Au(OH) 3 before its ultimate decomposition into O 2 47,50 . In summary, a series of water-soluble Au 25 NCs capped by three typical ligands were employed as a model to highlight the ligand effect on electrocatalytic OER kinetics. The RDS of OER for Au 25 NCs are altered by varying MHA and HCys to pMBA. Furthermore, Au 25 NCs capped pMBA with stronger electron withdrawing ability exhibit satisfied OER performance, significantly better than the other two ligand protected Au 25 NCs. The TOF of [Au 25 (pMBA 18 )] -NCs is4 to 5 times higher than other two Au 25 NCs. Mechanistic investigations revealed that ligands with stronger electron-withdrawing capability provoke more positive charges of active sites (i.e., Au(I) species) in Au 25 NCs. Such more partial positive charges play a key role in changing the RDS of OER. This work not only provides a molecular-level understanding towards the ligand effects on the electronic structures of active sites of metal NCs and the follow-up electrocatalytic performance, but also stimulates engineering strategy investigations in the field of cluster chemistry and electrocatalysis research.

Synthesis of [Au 25 (pMBA) 18 ] -
[Au 25 (pMBA) 18 ] -NCs were prepared using CO reduction according to the previous protocol with some minor amendments. In particular, 10 mL of aqueous solution of 50 mM pMBA (in 150 mM NaOH) and 5 mL of 50 mM HAuCl 4 were added into 238.75 mL of ultrapure water sequentially. Then, the mixed solution was stirred for 5 min. After that, the pH of the reaction mixture was brought up to 11.05 by dropping in 1.0 M NaOH aqueous solution and then was stirred for another 10 min to form Au(I)-(pMBA) complex. In sequence, CO was bubbled into the reaction mixture for 2 min to initiate the reduction of Au(I)-(p-MBA) complex. The mixed solution was allowed for proceeding air-tightly for 7 days at room temperature (25°C) and under vigorous stirring. The reddish-brown solution was obtained at the end of this procedure as raw products.  18 ] -NCs were prepared using NaBH 4 reduction according to the previous protocol with some minor amendments. In particular, 100 mL of aqueous solution of 5 mM MHA and 12.5 mL of 20 mM HAuCl 4 were added into 120 mL of ultrapure water for the formation of Au(I)-MHA complex. Then, 12.5 mL of 1.0 M NaOH solution was introduced into the reaction mixture, followed by the addition of 5 mL of 0.1 M NaBH 4 solutions. The mixed solution was stirred for 24 h and the raw products were collected. [Au 25 (HCys) 18 ] -NCs were synthesized using the same synthetic conditions except some minor changes in experimental parameters including the stirring time for 6 h.

Synthesis of Au nanoparticles (NPs)
Au NPs were prepared using NaBH 4 reduction. 12.5 mL of 20 mM HAuCl 4 and 5 mL of 50 mM pMBA were added into 120 mL of ultrapure water followed by the addition of 5 mL of 0.1 M NaBH 4 solutions. The precipitate was washed by water and ethanol for three times.

Purification
The raw products were first concentrated by 10 times via rotary evaporation (water bath temperature 45°C, cooling temperature 2°C, and rotation rate 102 r.p.m.). Ethanol (double the volume of the concentrated NC solution) was then added, followed by a centrifugation at 18,353 g. for 5 min. The resultant precipitate was washed with DMF for 2 times and re-dissolved in ultrapure water to form an aqueous solution of purified [Au 25 (SR) 18

Electrochemical experimental procedures
All electrochemical measurements were carried out using a standard three-electrode H type cell on Auto-lab electrochemical workstation at room temperature, with an Ag/AgCl as reference electrode and Pt foil as counter electrode. The half cells were separated by a piece of proton exchange membrane (Nafion 117) and filled with 1.0 M KOH as electrolyte. Before the OER experiment, the electrolyte was saturated with O 2 for 15 min. For the preparation of thin-film working electrode, the catalyst ink solutions were loaded on the carbon paper (1 cm 2 ) with a loading amount of 0.026 nmol Au 25 NCs. The catalyst ink solutions were prepared by dispersing certain amount of catalysts in N-Methyl-2pyrrolidone (NMP) with Nafion (5 wt%). Taking [Au 25 (pMBA) 18 ] -NCs as an example,~2 mg [Au 25 (pMBA) 18 ]powder and 2 mg of VulcanXC-72 were dispersed in 980 µL of NMP and followed by adding 20 µL of Nafion(5 wt%) and stirred this mixed solution until the formation of homogenous ink. Then, the electrode was dried in vacuum drying oven with 30 mbar and 60°C overnight for further measurement. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) analysis was carried out of 5 mV s -1 with iR correction.

In situ electrochemical-Raman tests
In situ Raman scatting spectroscopy measurements are carried out at room temperature using a micro-Raman spectrometer (Horiba JY-T64000) in a backscattering configuration. A solid-state laser (λ = 532 nm) and an Air ion laser (λ = 488 nm) have been used to excite the samples. The backscattered signal was collected through a 100× objective and dispersed by a 1800-g/mm grating.

Characterization
Solution and electrolyte pH was monitored by a Mettler Toledo FE 20 pH-meter. Crude product was centrifuged by an Eppendorf Centrifuge 5424. UV-vis spectra were recorded by a Shimadzu UV-1800 spectrometer with the fast scan speed and single scan mode. X-ray photoelectron spectroscopy (XPS) was conducted on a Kratos AXIS Ultra DLD spectrometer. All binding energies were referenced to the C(1 s) hydrocarbon peak at 284.5 eV. For the transmission electron microscopy (TEM), 10 μL aqueous solution of individual Au NCs (0.01 mM on Au atom basis) or Au NPs (~0.01 mg/mL) was dropped on the copper gird for TEM, followed by air-drying at room temperature. Cluster concentrations were measured and normalized by ICP-OES on a Thermo Scientific iCAP 6000 ICP-OES. ESI-MS analysis was carried out on a Bruker micro TOF-Q system in negative ion mode. Detailed operating conditions of ESI-MS analysis are given as followings: source temperature/120°C, dry gas flow rate/4 L per min, nebulizer pressure/ 0.4 bar, and capillary voltage/3.5 kV.

Computational details
Differential charge density maps: The spin unrestricted DFT calculations in this study are based on the package of DMol3 8.0 51,52 . The generalized gradient approximation (GGA) implemented in the Perdew-Burke-Ernzerhof (PBE) functional was employed as the exchange-correlation functional 53 . The DFT semi-core pseudopotentials 54 was chosen to describe the core electrons and the valence electrons were considered using a double numerical basis set including p-polarization function DNP (version 4.4) with the orbital cutoff of 4.5 Å. Dispersion corrected DFT (DFT-D) via the TS scheme was used to describe the dispersion interactions among all the atoms in models 55 . The electronic energy was considered selfconsistent when the energy change was smaller than 10 −6 eV. The solvation model COSMO 56 was used to simulate aqueous environment, with the dielectric constant set as 78.54 (which is a value for water). A 0.002 Ha smearing value was added to accelerate convergence.
The Au 25 NCs was put in a 30 Å × 30 Å × 30 Å cubic cell. The calculation was performed with a Gamma Monkhorst-Pack k-point grid. A geometry optimization was performed with the convergence criteria of the energy change smaller than 10 -5 eV, the gradient change smaller than 0.002 Ha/Å or the displacement change smaller than 0.005 Å. Atomic charges were obtained via the Mulliken method.

Adsorption energy profiles
Spin-polarized density functional theory (DFT) calculations were performed in the Quickstep module 57 of CP2K 58 . The mixed Gaussian and plane wave (GPW) approach 59 using the Perdew-Burke-Ernzerhof (PBE) functional 60 was employed with the Grimme D3-dispersion correction 61 . The Kohn-Sham orbitals were expanded in the molecularly optimized basis set with a double-ζ Gaussian basis set augmented with a set of p-type polarization functions (MOLOPT-SR-DZVP) 62 with core electrons represented by the Goedecker-Teter-Hutter pseudopotentials [63][64][65] . In all calculations, the plane-wave kinetic energy cutoff was set to 400 Ry. Convergence threshold of the electronic structure relaxation was set to 10 -6 Hartree and the force convergence criterion of the geometry optimizations was set to 4.5 × 10 -4 Hartree/Bohr.
We took the models of [Au 25 (SCH 3 ) 15 (SR) 3 ] 3−q (SR = pMBA or MHA, q = 0) to understand the effect of the ligands on the OER performance of Au 25 nanoclusters in alkaline conditions. In those models, three thiolate ligands were treated explicitly in their anionic form while others are capped by methyl groups. The charge state q = 0 of the Au cluster was chosen to be consistent with the experimental observation.
The computational hydrogen electrode was used to obtain the Gibbs free energy of OER for each elementary step in the form of ΔG = ΔE + ΔE ZPE -TΔS, in which ΔE is the difference of DFT total energy, ΔE ZPE the zero-point energy difference calculated from vibrational frequencies, and ΔS the entropy difference between the products and reactants for each step. The ZPE and entropy values were adopted from literature 66 .

Data availability
All relevant data are available from the corresponding authors on request.